2011: Year of the Tomato

Published: April 1, 2011

The National Garden Bureau selects one flower and one vegetable to showcase each year. This year the tomato is the vegetable of their choice. While the author has written several articles on tomato in recent years it hardly seems fitting to allow the notoriety bestowed this year on this staple of the home garden to go unnoticed. April is the month most avid gardeners in Missouri initiate the annual ritual of tomato production and a good time to give added homage to America's most popular home garden plant.

When contemplating this year's tomato planting one of the first things to consider is their growth habit. Determinate tomatoes are relatively compact, and reach a predetermined height or number of fruit clusters. Each short branch ends in a flower cluster, and plants do most of their growing before setting fruit. Determinate tomatoes tend to ripen in a short period, so that the main harvest is concentrated into a few weeks. This may be ideal for gardeners who wish to preserve fresh tomatoes for winter soups and sauces.

In contrast, indeterminate tomato plants grow, blossom, and produce tomatoes throughout the growing season. They can grow to over 10 feet tall and, if unpruned, produce many side stems, all of which are capable of flowering and fruiting. As shoot tips continue to grow, flower clusters are borne in the leaf axils of the elongating shoot. An example of an indeterminate variety is 1994 AAS winner 'Big Beef'. To help manage the robust growth of indeterminate varieties and keep their fruit off of the ground, supporting plants with cages, stakes or trellises is recommended. Staked plants should be pruned to remove all but one or two growing stems, which are tied loosely to the stakes and trained for vertical growth. Because this system allows air to circulate around the plants, it can help prevent disease infestation. Pruning also tends to produce larger (but fewer) tomato fruit. The process of pruning simply involves removing shoots (suckers) that grow from the point of leaf attachment (nodes) on the main stem.

There is a third type of tomato growth habit called semi-determinate. Varieties with this type of growth produce plants which are bushy like a determinate type, but will set and ripen fruit over a longer period of time. The 1984 AAS Award Winner 'Celebrity' is an example of a semi-determinate variety. The best way to grow determinate or semi-determinate plants is to not prune and place a cage around the tomato while still small. Alternatively, short stakes can be driven into the ground between every other plant in a row of tomatoes. Twine can then be "woven" between the tomatoes and wrapped around the stakes for support.

Tomatoes are sun-loving plants and need as much direct sunlight as possible to produce the highest yield. Native to South America, tomatoes require warm temperatures for good growth, so wait until the nighttime air has warmed to about 55 degrees F before transplanting them. Planting tomatoes too early will only slow their growth. Tomatoes are not frost hardy and will die if exposed to freezing temperatures without protection. If temperatures drop at night, keep young plants warm with a cloche or other type of cover.

Tomato plants grow well in many types of soil. Work the soil only when it is dry enough so it will not stick to tools. Improve garden soil by adding organic matter such as peat moss, leaf mold, well-rotted manure or compost. Tomatoes grow best in nearly neutral soil with pH of 6.5 to 7.0. If soil test results indicate the need for lime, add it in late fall or early spring.

Tomatoes are heavy feeders and need adequate amounts of nitrogen, phosphorus, and potassium for optimum performance in the garden. Start by supplying plants with an ample amount of these nutrients as a "pre-plant" fertilizer. Fertilizers relatively low in nitrogen but high in phosphorus and potassium (e.g. 6-24-24) are ideal. Use a maintenance rate of 1 pound per 100 square feet after the proper fertility level has been developed from previous soil tests and fertilizations. Organic sources of fertilizer also may be used.

No matter what type of fertilizer is selected, always follow the directions on the label and do not over fertilize. The latter this will cause lush vegetative growth with poor fruit set. Once tomato plants have set fruit additional nitrogen is needed in the form of a "side-dressing". The application of calcium nitrate every 10 days to two weeks after the first fruit are the size of a golf ball is a practice many gardeners follow. About 3.5 pounds of calcium nitrate can be side-dressed per 100 feet of row.

Tomatoes are subject to a number of diseases that can limit yield and reduce fruit quality. Most authorities recommend rotating tomatoes and other crops in the garden on a four-year cycle. This means not to grow the same crop (or a crop in the same family) in the same place more often than every forth year. Tomato, pepper, eggplant and potato all belong to the same plant family. Crop rotation helps to prevent the build-up of disease inoculum in the area immediately surrounding the tomato plant.

Additionally, when browsing through tomato seed packets or reading the care tag on a started plant the letters V, F, N, or T might be included on the description. These letters indicate the plant is genetically resistant to certain diseases, making further control unnecessary.

The letter 'V' stands for Verticillium wilt which is caused by a soilborne fungus. The symptoms of infection are wilting of older leaf tips, yellowing and browning of leaves in a V-shaped pattern and leaf drop beginning with the older foliage. As the fungus moves throughout the plant, all leaves curl upward and the stunted plant will not respond to water or fertilizer. Cool weather conditions encourage this disease.

The letter 'F' indicates the variety is resistant to Fusarium which also is a soil-borne fungal disease. Infection commonly occurs when the soil is above 75 degrees F. and plants located in light sandy soils, or soils with low pH, are most susceptible to Fusarium wilt. Symptoms of this disease are yellowing, curving and dying leaves; infected plants are stunted and fruits are small or deformed.

The letter 'N' stands for nematodes which are tiny round worms that live in the soil. Most nematodes are beneficial, but a few, including root knot nematodes, are plant parasites that cause stunted growth, wilting, and dieback. To verify this problem, pull the tomato from the soil. If the roots have growths or galls on them, root knot nematodes are the problem. Be sure to plant resistant varieties in the future.

Finally, the letter 'T' indicates the variety is resistant to tobacco mosaic virus which is a widespread tomato virus. Weeds harbor the virus and insects that feed on the weeds can transmit the disease to tomato plants. Symptoms are light and dark mosaic patterns on leaves, or yellow mottling. Tobacco, a close relative of the tomato, is the source of the virus, which can also be spread when smokers handle plants.

Other problematic tomato diseases for which resistance has not been found include early blight and Septoria leaf spot, either of which can be devastating to a crop of tomatoes. Early blight, caused by the fungus Alternaria solani, can be recognized by dark concentric rings on older leaves. Although spotted leaves usually die prematurely, the disease will not kill the entire plant. Because the fungus overwinters in residue, be sure to dispose of diseased plants properly.

Septoria leaf spot is caused by the fungus Septoria lycopersici. Symptoms include small, roughly circular spots with dark-brown borders and gray centers on leaves. Leaves may die and drop off if heavily infected. To prevent these two diseases start by avoiding crowding to promote good air circulation between plants. Also, when plants require water, keep foliage dry by watering the soil, not the leaves. Finally, the use of fungicides labeled for Alternaria and Septoria is usually required to keep plants blight-free.

There are a few minor fruit disorders that tomato growers often encounter. One of the most common is blossom-end rot. It begins with tan lesions on the blossom end of the tomato, which eventually enlarge into dark, sunken areas. This rot appears during periods of high growth or when soil moisture is alternately high and low. The direct cause is the fruits' inability to take up sufficient calcium. Maintaining uniform soil moisture by watering regularly and supplying the plant with adequate calcium will help to prevent this disorder.

Cracking is another disorder that occurs more in some varieties than others. Changes in conditions, such as high temperatures and irregular moisture levels can cause tomatoes to crack near the stem end. To prevent cracking, choose from among the many varieties that are more resistant to the disorder and keep soil moisture levels adequate and constant.

Problematic insect pests of tomato include tomato hornworm, cutworm, flea beetle, whitefly and aphid. Regular inspection followed, if necessary, by early intervention can minimize insect damage on tomato plants and fruits. If infestation levels warrant chemical control, use only those pesticides labeled for use on food crops and start with those having lowest mammalian toxicity. When using any pesticide, always read and follow label directions.

In addition to being very tasty, tomatoes are an excellent source of vitamins and minerals. One cup of cherry tomatoes will provide 25% of daily recommended Vitamin A, 32% of Vitamin C, and a substantial amount of Vitamin K and potassium. Tomatoes are also an excellent source of lycopene, a powerful antioxidant that has been linked to a reduced risk of cancers. For the best tasting, most nutritious tomatoes, grow them yourself and allow them to ripen on the vine.